Stone fruit, like apricot, nectarine, plum, cherry and peach are grown in orchards. Under average growing conditions, pollination (self or cross) is very efficient and many fruits are obtained. However, the fruit carrying capacity of trees is limited and, hence, the increase of number of fruit is accompanied by reduction in fruit size. Since small fruit is qualified in the fresh fruit market as low quality fruit and are directed to industry at very low prices, it is common practice among fruit growers to reduce the number of fruits per tree by chemical, mechanical and/or hand thinning in order to prevent branch breakage, and to obtain large, high quality fruits. Also, abundant bearing may cause bi-annual bearing, which is disadvantageous by itself.
The need for fruit load control is widely recognised in stone fruit. Research on chemical fruit thinning has had limited success so far. Therefore growers are left with very few options: (i) flower hand thinning which requires a lot of man power and is very expensive, (ii) fruit hand thinning which is typically done 4-6 weeks after full bloom and requires a lot of man power and this is therefore very expensive, (iii) mechanical flower thinning which is done during flowering; but this has some downsides: difficulty to evaluate the thinning intensity, risk of tree damages, not good repartition of removed flowers, which results in difficult fruit coloration, or additional manually removing of leaves to let the light inside the canopy. Therefore, grower practice for the moment can be summarised as follows: (i) On some varieties, mechanical flower thinning is done in order to remove part of the flowers and to limit hand thinning later on. Depending on the crop/variety, this can be 50-150 h/ha (h/ha=hours of hand-thinning per ha). This is mainly done on very fertile peach/nectarin varieties like Carène, Gardeta, Garcica, Ivory Star, Lorinda, Early Top, and Valley Sweet. It is used also on young trees (3rd leaf). (ii) On all varieties, fruit thinning is done by hand in the early stages of fruit development. This is estimated to represent 50-250 h/ha depending on the varieties. Thus, together, the work load for flower+fruit thinning is commonly estimated between 200 and 250 h/ha in average but can reach up to 400 h/ha. Because of the high labour cost, chemical thinning operations is sought for, provided it gives stable results.
A number of chemical compounds have been suggested to be suitable for thinning stone fruit. Examples of suggested chemical thinning agents are e.g. described in:                U.S. Pat. No. 2,957,760, alpha-(3-chlorophenoxy)-propionic acid and its salts;        GB1509195, combinations of auxin, gibberellin 3 and a urea-derivative;        EP089205, 2-pyrrolidone-5-carboxilic acid, N-mixed fatty acylated L-glutamic acid sodium salt or fatty acylated sugar molecules;        EP463241, certain alkoxylated amines;        U.S. Pat. No. 5,622,915, 3,5,6-trichloro-2-pyridyloxyacetic acid or salts thereof        US2012/088668, anthranilic acid and/or acetophenon, optionally in combination with auxins        
In practise, only very few products are available for stone fruit. These products include (i) GA3 based products. In Spain, a 3.6% GA3 formulation is used: Ralex from Kenogard and Laikuaj from Cequisa on an authorised rate of 0.18-0.25%. The authorised use is to improve fruit size and reduce flowering in peach. The timing of treatment indicated in the registration notice is 4 and 2 weeks before harvest. Ralex is also authorized in Australia for use in stone fruit (apricots, nectarin and peach, at 70-400 ml/100 for a 40 g/L GA3 formulation applied from early December to late January); (ii) GA3 based products (Berelex and Gibb 3) are authorised in France to modify fruit set level in peach. Dose is 4-6 tablets/100 (4-6 g/L) for Berelex applied during floral induction. (iii) NAA (naphthalene acetic acid, a synthetic auxin) is used in Italy (Fixormon: 85 g/L NAA) and is authorised for peach thinning at 12-20 mL/100 with an application timing when fruitlet are 10-15 mm in diameter.
Of all the suggested chemical treatments, only the treatment with GA3 (gibberellic acid) is relatively common. GA3 is also available as tablets (Falgro). Literature references are for example: Southwick et al. J. Amer. Soc. Hort. Sci. 120(6) (1995) 1087-1095; Southwick & Glozer, Hortechnology, 10(4) (2000); and González-Rossia et al. Scientia Horticulture 111 (2007) 352-357.
Southwick in the 26th Proceedings PGR society of America, (1999), pp 50-55 also reports on the use of Ralex (GA3) on several stone fruit cultivars. Accidentally, the paper comprises one table wherein effect of GA3 is compared with GA4 and with GA7 when applied on an apricot variety in May or June. The source and purity of GA4 or GA7 are unclear. Further, the paper explains that GA4 is more effective than either GA3 or GA7. GA7 shows only in 1 out of 6 experiments a non significant reducing effect on flowering and in 4 experiments out of 6 shows the opposite effect of increased flowering (significant vs. untreated).
Oliveira and Browning in Plant Growth Regulation (1993) 13:53-63 report on effects on flower initiation of a number of gibberellins. These GAs are applied as 10 μl of a 60% aqueous ethanol solution between the points of bud insertion in the 6-8 consecutive distal areas of individual fruiting spurs, i.e. on the shoots between the buds.
Acta Horticulturea (2000) comprises the Proc. Of the Int Conf on integrated fruit Production (1998), 525:467-469, XP-002720822, and 525:463-466, XP-002720823, wherein Singh et al. describe experiments on GA4-GA7 mixtures, which mixtures are generally about 50/50. The treated crop Phalsa was sprayed at fully swollen flower bud stage and none of the treatments inhibited the return bloom in this crop.
Despite these and other experiments, no suitable alternatives are known to date.
Hence, there is a clear need for a further improved stone fruit thinning agent.